This invention relates to production of protein containing paper coating compositions and particularily those containing modified vegetable protein as the adhesive binder. Applicant is aware of the following U.S. Patents the disclosures of which are incorporated by reference herein.
U.S. Pat. Nos. 2,274,983 2,246,466 2,849,334 2,881,076 2,950,214 3,081,182 3,411,925 4,048,380 4,109,056 4,277,524 4,352,692 4,421,564 4,474,694 4,520,048 4,575,477 4,581,257 4,607,089 4,620,992 4,812,550
Vegetable protein binders are well known as adhesive binders for paper coating compositions. In particular, modified soy protein has found extensive use as an adhesive binder in paper coating compositions. These adhesive binders are typically prepared from isolated soy protein extracted from oil free soybean flakes in an alkaline solution and recovered by isoelectric precipitation. The recovered soy protein is then typically modified by hydrolysis and other chemical treatment to prepare a protein adhesive material suitable for use as a binder in paper coating compositions. These binders find wide usage in the paper coating industry.
However, in spite of their wide acceptance, these materials still suffer some disadvantages, particularly when compared with casein and other synthetic containing adhesive binders for paper coating compositions.
In particular, soy protein binder solutions and coating compositions containing soy protein binders have in the past been handicapped by a lack of stability at room temperature. As a result it is often not possible to prepare large quantities of these materials suitable for long paper coating runs, since the color and properties of the paper coating composition will deteriorate as a result of the bacterial attack on the protein binder itself. Moreover, paper coating compositions prepared from these adhesives have in the past also suffered from reduced whiteness, that is by having a yellow undertone present and from a lack of gloss and optical brightness in the prepared paper coating composition.
Applicants have found however, that if conventional soy protein materials are subjected to an oxidation treatment, an unexpected increase in the stability and life of paper coating compositions prepared from the adhesives will result. The oxidizer may be a conventional strong oxidizing material, such as hydrogen peroxide or an equivalent oxidizer. Typically the adhesive material is treated in the wet state, for example as a wet curd recovered from the isolation or hydrolysis process. The conditions of oxidation are not critical, but for hydrogen peroxide a typical addition would be between about 3% to 25% hydrogen peroxide (70% solution) based on the weight of the dry solids. Oxidation would typically be carried out at slightly elevated temperatures, for example, from about 30.degree. C. to 70.degree. C., for example 60.degree. C. for times from about 1/2 hour to 6 hours, preferably about 90 minutes.
In addition to providing an unexpected increase in the shelf life of the paper coating compositions prepared from the binder, the binder also imparts significantly improved properties in terms of the overall optical characteristics of the coated paper. Use of this adhesive binder can increase the whiteness of the coatings prepared from it to levels equal to or better than those obtained by casein, an industry standard. The gloss and the brightness of the paper coatings using applicants' compositions are exceptional.
It has also been unexpectedly found that the solubility characteristics of the new adhesive binder are such that adhesive binder solutions, and thus paper coating compositions, can be prepared at much greater percent solids, thus enabling paper producers to realize a substantial savings in the energy cost required to drive the water of suspension out of the paper coating compositions. These high solids adhesive binder solutions can be prepared both using cold (room temperature) water and using water which has been heated. In either case, considerable increase in the dispersability of the adhesive binders is achieved.
It is thus an object of applicants' invention to prepare a soy protein, adhesive binder, and a paper coating composition produced therefrom, which has an increased room temperature storage life when formed into paper coating compositions.
It is an object of applicants' invention to prepare an adhesive binder from soy protein which has the ability to produce high solids coating compositions, when prepared hot or cold.
It is a further object of applicants' invention to prepare an adhesive binder and a paper coating composition which has room temperature shelf life and storage stability suitable for use in long coating runs.
It is a further object of applicant's invention to produce an adhesive binder which has increased whiteness, brightness and gloss.
These and other objects of the invention will be understood from the following description of the preferred embodiments.